This application claims the priority of Japanese Patent Application No. 10-101978 filed on Mar. 30, 1998, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light-scanning optical system; and, in particular, to a light-scanning optical system equipped with a function of correcting, when a luminous flux emitted from a light source is reflected and deflected by light-deflecting means such as polygon mirror so as to scan a surface to be scanned, the unevenness in pitches caused by the surface tilt of the light-deflecting means.
2. Description of the Prior Art
There have conventionally been known light-scanning optical systems such as laser printer and digital copier which scan a surface to be scanned such as a photosensitive material with a light beam such as laser beam so as to form an image on this surface. Such a light-scanning optical system comprises light-deflecting means such as rotary polygon mirror for reflecting and deflecting the light beam emitted from the light source so as to scan the surface to be scanned, and a scanning and imaging optical system for forming an image of the light beam onto the surface to be scanned.
In such a light-scanning optical system, when an unevenness in inclination (surface tilt) exists among the deflecting/reflecting surfaces of the light-deflecting means with respect to the surface to be scanned, then the imaging position of the light beam may fluctuate among scanning lines in the direction (sub-scanning direction) substantially orthogonal to the main scanning direction on the surface to be scanned, thus generating an unevenness in pitches, whereby favorable images may not be obtained. Therefore, various kinds of light-scanning optical systems equipped with a function of correcting the surface tilt of the deflecting means have been known.
For example, there has been known a light-scanning optical system equipped with a function of correcting the surface tilt, in which, in first optical means, a luminous flux from a light source is made substantially parallel by a collimating lens and then is condensed along the sub-scanning direction in the vicinity of a deflecting/reflecting surface of light-deflecting means by a cylindrical lens or the like so as to form a linear image along the main scanning direction, and the luminous flux deflected and reflected by the deflecting/reflecting surface is focused by second optical means located downstream the light-deflecting means onto the surface to be scanned. In the light-scanning optical system equipped with this surface-tilt-correcting function, an fxcex8 lens system is widely used for the second optical means and has a function of focusing the luminous flux from the light source as a light spot onto the surface to be scanned and a function of moving this light spot at a constant speed on the surface to be scanned. Such second optical means is often constituted such that the position of the deflecting point on the deflecting/reflecting surface and the position of the imaging point on the surface to be scanned are substantially conjugate with each other in the sub-scanning direction.
In recent years, there have been demands for such a light-scanning optical system having a surface-tilt-correcting function to reduce the number of components and facilitate its manufacture.
For example, in the light-scanning apparatus disclosed in Japanese Unexamined Patent Publication No. 6-18803, a single spherical lens or an aspherical plastic lens, a planoconvex cylindrical lens having a positive refracting power only in the main scanning direction, and a concave cylindrical mirror having a negative refracting power only in the sub-scanning direction are disposed between light-deflecting means and a surface to be scanned, whereby the surface tilt is corrected with a small number of components which are easy to process.
Recently, in addition to such simplification of the structure, there have also been demands for attaining a smaller size in the light-scanning optical system, or a higher density in its forming image quality so as to improve the accuracy of image. Also, there is an important demand for reducing the cost. However, it is difficult to realize these demands at the same time.
For example, in order to reduce the size of the light-scanning optical system, it is required for the scanning lens, even in its peripheral portion, to minimize its aberration and have a wide scanning angle, whereby the scanning lens tends to become an optical system composed of a large number of elements.
The higher density in image quality specifically refers to the fact that the demanded performance has recently shifted to a printing function on the order of 1000 dots per inch from the conventional function on the order of 600 dots per inch. Thus, the conventional light-scanning optical systems have been failing to respond to such a demand. It is critical to a higher density in image quality that the size of the light spot be made smaller on the surface to be scanned. As a means therefor, a lens having a large aperture may be used. The resulting configuration contradicts the reduction in size of the optical system, and it is difficult to lower the cost thereof since the scanning lens becomes complicated.
On the other hand, it has been proposed to use plastic lenses so as to reduce the number of lenses or simplify the configuration in general. This is also advantageous in lowering the cost. Nevertheless, since favorable performances are hard to attain in the peripheral portion of a plastic lens, it is difficult to broaden the aperture relative to the lens diameter. Hence, it becomes difficult to achieve a smaller size when the aperture is broadened so as to attain a higher density as mentioned above.
Also, while the above-mentioned light-scanning apparatus disclosed in Japanese Unexamined Patent Publication No. 6-18803 uses an anamorphic mirror or lens such as cylindrical lens for correcting the surface tilt, such a mirror or lens is positioned relatively near the surface to be scanned in order to keep the image surface curvature favorable in the sub-scanning direction. As a consequence, the mirror or lens has a form elongated in the main scanning direction. As the mirror or lens that is hard to process becomes larger, the product is more likely to increase its cost. Hence, there is a demand for further simplifying the optical system and lowering its cost.
In view of such circumstances, it is an object of the present invention to provide a light-scanning optical system which has a surface-tilt-correcting function with a small number of constituent lens elements and can respond to higher image density.
It is another object of the present invention to provide a light-scanning optical system which can reduce its size and cost while using a plastic lens.
A first light-scanning optical system in accordance with the present invention comprises:
a light source;
first optical means for forming a linear image of a light beam from the light source;
light-deflecting means, having a light-deflecting surface at or near an imaging position of the first optical means, for deflecting the light beam so as to scan a surface to be scanned; and
second optical means for causing the light beam deflected by the light-deflecting means to form an image on the surface to be scanned and scan the surface at substantially a constant speed;
wherein the second optical means is constituted, successively from the light-deflecting means side, by:
a first lens having a positive refracting power on each of main scanning and sub-scanning directions, each of both sides thereof having a toric surface; and
a second lens having a small negative refracting power or no refracting power in the main scanning direction, at least one surface thereof being made of a non-arc surface whose cross section in the main scanning direction is represented by a function of higher order.
Alternatively, at least one surface of the second lens may be made of an aspheric surface which is of rotational symmetry.
Preferably, a semiconductor laser having a wavelength of 500 nm or shorter is employed as the light source.
A second light-scanning optical system in accordance with the present invention comprises:
a light source using a semiconductor laser having a wavelength of 500 nm or shorter;
first optical means for forming a linear image of a light beam from the light source;
light-deflecting means, having a light-deflecting surface at or near an imaging position of the first optical means, for deflecting the light beam so as to scan a surface to be scanned; and
second optical means, including at least one plastic lens, for causing the light beam deflected by the light-deflecting means to form an image on the surface to be scanned and scan the surface at substantially a constant speed.
Here, the above-mentioned xe2x80x9cmain scanning directionxe2x80x9d refers to a direction parallel to the locus of the deflected light beam on the surface to be scanned, the above-mentioned xe2x80x9csub-scanning directionxe2x80x9d refers to a direction substantially orthogonal to the main scanning direction on the surface to be scanned, and the above-mentioned xe2x80x9ccross section in the main scanning directionxe2x80x9d refers to the cross section in the main scanning direction including the optical axis.
The above-mentioned xe2x80x9ctoric surfacexe2x80x9d refers to a surface in which the cross section in the main scanning direction and the surface (sub-scanning direction), perpendicular to the cross section in the main scanning direction, including the optical axis have refracting powers different from each other.